Studies on the hepatic excretion of organic anions are being performed both in vitro and in vivo. Ether anesthetized, acutely nephrectomized rat comparisons of the biliary excretion of iopanoic acid (a widely used cholecystographic agent) and its only excretory product, iopanoic acyl-mono-glucuronide, have indicated that the rate of glucuronidation of iopanoate is the rate-limiting step in its overall excretion. This is in contrast to the rate-limiting step for the excretion of p-nitrophenol where the data demonstrate that the transport process responsible for the movement of p-nitrophenol glucuronide (the only excretory product following p-nitrophenol) from the hepatocyte to the bile is rate-limiting. A third organic anion, phenolsulphophthalein appears to be excreted by two parallel routes, as a glucuronide and as the administered compound. Glucuronide excretion is saturable, whereas the excretion of the parant compound is not. It is not clear if the intrahepatic glucuronidation of phenolsulphophthalein is the limiting step to the excretion of the glucuronide. In order to evaluate the possible influence of intracellular binding to potential transport proteins on the overall biliary excretory process experiments are being undertaken to isolate ligandin (an organic anion binding protein) to measure binding constants for the various organic anions studied in vivo. These studies will provide a basis for the potential physiological role of this protein and possibly provide evidence with which to explain the observed competition for excretion between the various anions.